[PATCH] D91017: [BranchProbabilityInfo] Use SmallVector (NFC)

[Kazu Hirata via Phabricator via llvm-commits]

kazu added a comment.

In D91017#2381145 <https://reviews.llvm.org/D91017#2381145>, @MaskRay wrote:

> It is true that SmallVector is more convenient to use but this change also has potential to increase memory usage. Have you checked whether this can increase memory usage?

The differences in memory consumption seem to be really small.

I have invoked clang for three input files, three times each, and looked at `Maximum resident set size (kbytes)` with `/usr/bin/time -v`.

  JumpThreading.ii
  Without this patch: 280792 281016 281392
  With this patch:    280736 281484 281568
  
  SROA.ii
  Without this patch: 311820 312412 312488
  With this patch:    311224 311356 311468
  
  LICM.ii
  Without this patch: 346300 346380 348356
  With this patch:    345528 348084 348564

Under the hood, the difference should be a wash because:

  DenseMap<std::pair<const BasicBlock *, unsigned>, BranchProbability>::value_type

is 24 bytes in size, padded to 32 bytes, and

  DenseMap<BasicBlock *, SmallVector<BranchProbability, 2>>::value_type

is 32 bytes in size.

If we assume that we have a function with 100 "if" statements with no switch statements at 50% occupancy of the hash table, we have:

  Without this patch: 32 bytes/entry * 200 edges        / 50% occupancy = 12,800 bytes
  With this patch:    32 bytes/entry * 100 basic blocks / 50% occupancy =  6,400 bytes


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D91017